1. Field of the Invention
The present invention relates to a vector quantizer for quantizing the vector of image information and audio information.
2. Description of the Prior Art
The prior art relating to such a vector quantizer is described in literatures such as: Y. Linde, A. Buzo and R. M. Gray, "An Algorithm for Vector Quantizer", IEEE Trans., Com-28, pp. 84-95 (1980); A. Gersho, "On the Structure of Vector Quantizers", IEEE Trans., IT-82, pp. 157-166 (1982); A. Buzo and A. H. Gray, Jr, "Speech Coding Based Upon Vector Quantization", IEEE Trans., ASSP-28 (1980); B. H. Juang and A. H. Gray, Jr., "Multiple Stage Quantization for Speech Coding", Proc. Intl. Conf. on A.S.S.P., pp. 597-600, Paris (1982); and A. Gersho and V. Cuperman, "Vector Quantization: A Pattern Matching Technique for Speech Coding", IEEE, COMMUNICATION MAGAZINE, pp. 15-21, Dec. (1983).
A vector quantizer as shown in FIG. 3, which will be described later, is known in addition to those vector quantizers of the prior art. The principle of the vector quantizer will be described prior to the description of the vector quantizer of FIG. 3. Referring to FIG. 1, assuming that an object has moved on the screen during the period from a frame No. f-1 to a frame No. f from a position A to a position B, then a block S.sup.f (R) including a plurality of the lattice samples of an image signal around a position vector R in the frame No. f becomes approximately equal to the block S.sup.f-1 (R-r) of the image signal at a position determined by subtracting a movement vector r from a position vector R in the frame No. f-1. As shown in FIG. 2, when the position vector R=(m, n) and the movement vector r=(u, v), S.sup.f (R).apprxeq.S.sup.f-1 (R-r). Suppose the image signal at R=(m, n) is S(M, n) and S.sup.f (R)=[S(m-2, n-2), . . . , S(m, n), . . . , S(m+2, n+2)], the degree of analogy L(n, v) between S.sup.f (R) and S.sup.f-1 (R-r) is defined as the block unit matching scale of 5.times.5 picture element as follows; ##EQU1## In this state, the movement vector r is expressed by ##EQU2## That is, the degree of analogy L(u, v) of S.sup.f (R) and S.sup.f-1 (R-r) is expected to be minimized by block matching. Therefore, in introducing movement compensation into inter-frame prediction coding, the use of a block which minimizes L(u, v), extracted from image signal blocks of the frame No. f-1, as a prediction signal in giving the block S.sup.f (R) of the image signal of the position of R in the frame No. f to an inter-frame prediction encoder, the power of prediction error signal will be reduced to the least and encoding efficiency is improved.
Referring to FIG. 3 showing an example of the constitution of a conventional vector quantizer of this kind, there are shown an A/D converter 1, a raster/block scan converter 2, a frame memory 3, a movement vector detector 4, a variable delay circuit 5, a subtractor 6, a scalar quantizer 7, an adder 8 and a variable-length encoder 9.
The manner of operation of this vector quantizer will be described hereinafter. First the A/D converter 1 converts an analog picture input signal 101 into the corresponding digital signal and gives a digital picture signal sequence 102 according to the sequence of raster scanning. The output procedure of the raster scan digital picture signal sequence 102 on the time series of the picture signal is converted into block scanning by the raster/block scan converter 2, and thereby the digital picture signal sequence is converted into a block scan picture input signal 103 arranged sequentially in lattice block units (the interior of the block is raster scanning) from the top to the bottom and from the left to the right on the screen. A regenerative picture signal 104 of one frame before regenerated according to interframe DPCM loop is read from the frame memory 3. The movement vector detector 4 executes the block matching of the present block scan picture input signal 103, the regenerative picture signal 104 of one frame before and the picture signal and gives the movement vector r=(u, v) of the picture signal 104 of one frame before which minimizes the degree of analogy. The movement vectors (u, v) correspond to the horizontal and the vertical shifts of the picture element of the block of the regenerative picture signal 104 of one frame before. On the basis of the movement vector, the variable delay circuit 5 block-shifts the one-frame preceding regenerative picture signal 104 by the movement vector and gives a predictive picture signal 106 which is the closest to the present block scan picture input signal 103. The subtractor 6 calculates the differential between the block scan picture input signal 103 and the predictive picture signal 106 and gives a predictive error picture signal 107 to the scalar quantizer 7. The scalar quantizer 7 converts the predictive error picture signal 107 into a predictive error quantization picture signal 108 reduced in quantization level at picture element unit. The adder 8 adds the predictive error quantization picture signal 108 and the predictive picture signal 106 and gives a regenerative picture signal 109 including scalar quantization error to the frame memory 3. The frame memory performs delaying operation to delay the present regenerative picture signal 109 by one frame. In the movement compensation interframe DPCM loop, supposing that the picture input signal 103 is S.sup.f (m, n), the predictive picture signal 106 is P.sup.f (m, n), the predictive error signal 107 is .epsilon..sup.f (m, n), scalar quantization noise is Q.sub.s.sup.f (m, n), the predictive error quantization signal 108 is S.sup.f (m, n) and the one-frame preceding regenerative picture signal 104 is S.sup.f-1 (m, n), then EQU .epsilon..sup.f (m, n)=S.sup.f (m, n)-P.sup.f (m, n), EQU .epsilon..sup.f (m, n)=.epsilon..sup.f (m, n)+Q.sub.s.sup.f (m, n), EQU S.sup.f (m, n)=P.sup.f (m, n)+.epsilon..sup.f (m, n)=S.sup.f (m, n)+Q.sub.s.sup.f (m, n), and EQU S.sup.f-1 (m, n)=S.sup.f (m, n).multidot.Z.sup.-f.
where Z.sup.-f denotes delay corresponding to one frame.
P.sup.f (m, n) is expressed through movement compensation on the basis of S.sup.f-1 (m, n) by the following formula: EQU P.sup.f (m, n)=S.sup.f-1 (m-u, n-v).
FIG. 5 shows an example of the constitution of the movement vector detector 4 for carrying out movement compensation.
Referring to FIG. 5, there are shown an analogy degree computation circuit 10, a movement region line memory 11, a line memory control circuit 12, an analogy degree comparator 13 and a movement vector latch 14.
The movement vector detector 4 gives S.sup.f (R) produced by blocking a plurality of the sequences of the present picture input signal 103 to the analogy degree computation circuit 10. At this moment, the lines of the one-frame preceding regenerative picture signal 104 stored in the frame memory 3 corresponding to the tracking range of the movement region of S.sup.f (R) are stored in the movement region line memory 11. The line memory control circuit 12 sends sequentially the blocks adjacent to a plurality of blocks S.sup.f-1 (R+r) of the one-frame preceding regenerative image signal 104 to the analogy degree computation circuit 10. The analogy degree computation circuit 10 computes the analogy degree L(u, v) of the blocks in the neighborhood of S.sup.f (R) and S.sup.f-1 (R-r) and the analogy degree comparator 13 determines the minimum analogy degree min L(u, v). Since u and v correspond to the horizontal and the vertical address shifts of blocks in the movement region line memory 11 respectively, the analogy degree comparator 13 gives a movement detection strobing signal 111 to the movement vector latch 14 when the analogy degree is minimized, to take in a movement vector address 112. The movement vector latch 14 sends the displacement r of S.sup.f-1 (R-r) relative to S.sup.f (R) minimizing the analogy degree L(u, v) as a movement vector 105 to the variable delay circuit 5 and the variable-length encoder 9 of FIG. 3.
The variable-length encoder 9 of FIG. 3 processes the movement vector 105 and the predictive error quantization signal 108 through variable-length encoding to reduce the amount of information of the picture signal. The variable-length encoding enables the transmission of the movement compensation inter-frame encoding output 110 at a low bit rate.
Since the conventional movement compensation inter-frame encoder is constituted as described hereinbefore, the movement compensation operation is performed for every block and the inter-frame DPCM operation is performed for every picture element. Accordingly, the identification between the minute variation of the screen and noise is impossible and the variable-length encoding of the movement vector and the predictive error quantization signal is difficult. Furthermore, since the control of the variation of the quantity of produced information resulting from the variation of the movement scalar is difficult, a large loss is inevitable when transmission is carried out through a transmission channel of a fixed transmission capacity. Still further, encoding the predictive error quantization signal for every picture element is inefficient. Since the movement compensation system is liable to be affected by transmission channel error, the frame memory needs to be reset or the transmission repeated when transmission channel error occurs, which requires long resetting time.